Natural fractures are common in both reservoir and non-reservoir rocks. The quantification of the fracture properties is a key step in the development of reservoirs containing fractures. This quantification includes orientation, distribution (or density) and the openness (or aperture) of the fractures. One of the most common techniques for understanding the distribution and properties of fractures is to use electrical images of the borehole. The responses of fractures on electrical borehole imaging tools have been recognized since soon after the introduction of the early imaging tools in the 1990's. In the case of open fractures, and a normal overbalanced drilling condition with water base mud, the fractures appear conductive. The excess conductivity (or more correctly excess current flow) relative to the background response has a close relationship with the aperture of the fracture. Work published in 1990 by Luthi and Souhaité (Luthi, S. M. and Souhaité, P. “Fracture Apertures From Electrical Borehole Scans.” Geophysics 55, 1990. 821-833) demonstrated a relationship between the aperture of the fracture and a function of the excess current caused by the fracture, the background resistivity of the rock and the resistivity of the fluid filling the rock. This is expressed as an equation (commonly known as the Luthi-Souhaité equation):W=c·A·Rmb·Rxo1-b  (1)Where:                W=fracture aperture (mm);        A=excess current (μA mm/V);        Rm=mud resistivity (ohm·m);        Rxo=background formation resistivity (ohm·m);        b and c are parameters relative to the tool.        
Equation (1) is widely used for the calculation of the apertures of fractures. It is based on hypothesis relative to the formation or the tool, such as the following:                The response on the image log is due to a single fracture.        Fractures are perpendicular to the wellbore.        Fractures are of infinite length and constant width.        Fractures are filled with one fluid of known resistivity, whole mud resistivity (Rm) by default.        Scaling of imaging tool response to resistivity is valid.        Matrix is of uniform material.        Invasion from the fracture wall is insignificant.        
In view of the above, the confidence levels associated with the calculated outputs of the Equation (1) vary from one well to another or even to one zone of the well to another.